Baseband injected pilot carriers are often used in communication systems to enhance the performance of required operations in a high performance modem. An example of such system is disclosed in commonly assigned U.S. Pat. No. 6,606,357, the disclosure which is hereby incorporated by reference in its entirety. The '357 patent discloses a QPSK modulation scheme that uses a data spreading mechanism to take a relatively limited portion of the available transmitter power and inject into the QPSK waveform a prescribed amount of carrier signal power. Detection and non-regenerative extraction of the carrier at the receiver can be achieved without incurring a signal-to-noise degradation penalty. This injected, carrier based modulation scheme can use high performance, forward error correction (FEC) coding to reduce the signal power required for achieving a low energy per bit-to-noise density ratio (Eb/No) on the order of 1 to about 0 dB.
The price paid for this type of improvement in tracking performance is either (1) added bandwidth when a non-constellation symbol is used, or (2) a reduced throughput when the baseband pilot injected carrier (BPIC) symbols are part of the data constellation and are time multiplexed with payload data. It is preferred, however, to have a baseband injected pilot carrier based system that provides performance enhancements, without increasing bandwidth or decreasing the payload throughput.